The present disclosure relates generally to surface-modified glass substrates, and more particularly to glass substrates having a scratch-resistant layer provided over a major surface of the substrate.
Scratches are a concern for glass cover applications in hand held devices and other devices such as monitors and other displays. Scratches increase the scattering of light and can reduce the brightness and contrast of images and text that is presented on such screens. Further, in the device-off state, scratches can make the display look hazy, marred and unattractive. In particular for displays and handheld devices, scratch resistance can be an important attribute.
Scratches can be characterized by their depth as well as their width. Deep scratches extend at least 2 microns into the surface of the material, and wide scratches are more than 2 microns wide. Due to the physical extent of the scratch, fragmentation or chipping typically accompanies deep and/or wide scratches. In brittle solids, though, such as glass substrates, the resistance to deep and wide scratches can be improved through optimization of glass chemistry, i.e., glass composition.
On the other hand, scratches can also be shallow and/or narrow. Shallow scratches are characterized by a depth of less than 2 microns, and narrow scratches are characterized by a width of less than 2 microns. Scratches at these dimensional scales are sometimes described as “microductile” scratches. In displays and handheld devices, where a glass cover can be formed from an oxide glass, a large fraction of the scratches accumulated during use are believed to be microductile scratches. Though microductile scratches are not typically associated with large volumes of fragmented or chipped material, microductile scratches can adversely affect the optical properties of a glass cover. Further, in contrast to the larger, “heavy” scratches, microductile scratches are not easily prevented through modification of the glass chemistry.
The formation of microductile scratches can be attenuated by adjusting the hardness of the surface that is being scratched. Harder surfaces typically are more resistant to microductile scratching. While oxide glasses that form the glass substrates used in many glass covers typically have hardness values in the range of 6 to 9 GPa, as disclosed herein, the propensity of microductile scratch formation can be dramatically decreased by forming a harder surface layer on the oxide glass.
In view of the foregoing, it would be desirable to provide a hard, optically-transparent, scratch-resistant layer that can be applied to rigid glass covers that is economical, and physically and chemically compatible with the underlying glass.